The present invention relates to a fusion protein of a component of an internalizing receptor system and a moiety that binds to a specific cellular surface marker on a cell, to a conjugate of a toxin and a ligand for the internalizing receptor system, and to a method of tumor therapy using the conjugate and internalizing receptor system.
There is now a fairly large and growing body of experience in the use of monoclonal antibodies (mAbs) for the therapy of lymphoma. Several studies targeting different B-cell restricted CD (clusters of differentiation) antigens have shown promising results. These studies have used radiolabeled mAbs and, to a lesser extent, mAb-toxin conjugates, and have targeted CDs19-22, CD37, and HLA-DR.
MAbs used in lymphoma therapy differ in their ability to bind cognate antigen and to become internalized. For example, CD22 exhibits efficient internalization as well as reexpression of antigen after internalization. It suffers, however, from relatively low expression levels on most B-cell malignancies, and is not widely expressed, e.g., it is expressed on only 30-50% of cases of B-cell lymphocytic leukemia (B-CLL).
The present inventor has studied an anti-CD22 mAb, LL2. Preliminary studies using LL2 labeled with .sup.131 I for both therapy and imaging of NHL have produced response rates of 30-90+%, with varying percentages of complete responses and differences in durability of response. Higher response rates and longer disease-free survival have been associated with higher total doses of antibody and of radioactivity, which usually have required autologous bone marrow or peripheral stem cell rescue. While the results are encouraging, it is desired to increase therapeutic efficacy and decrease toxicity, particularly myelotoxicity.
The CD20 antigen, in contrast to the CD22 antigen, is a quite highly expressed B-cell restricted antigen that is expressed on a wide range of B-cell malignancies, ranging from acute lymphocytic leukemia (ALL) to the more differentiated B-Cell (B-CLL) and non-Hodgkin's lymphoma (NHL), and even to hairy cell leukemia (HCL). It generally is expressed on cells in the vast majority of cases of these malignancies at a high antigen density. A major disadvantage of CD20 is that it is a slowly internalizing antigen. For RAIT directed against CD20 this feature may not be a problem, but it militates significantly against the use of CD20 for toxin-based therapy.
A further problem of CD20 is the fact that B-cell malignancies exhibit a more rapid dissociation of bound anti-CD20 mAbs from the surface as compared to nonlymphoma tumor cells. This suggests that a therapy that uses bonding to a B-cell restricted antigen, particularly those characterized by slow internalization, would not be successful.
A variety of mAb-toxin constructs have been tested in both in vitro experiments and human trials. These studies have demonstrated potent and specific effects of these reagents. Most of the toxin molecules that have been used derive from either plant or bacterial sources and hence produce allergenic sensitization in patients. This severely limits the duration of therapy.
While major progress has been made in the therapy of B-cell malignancies such as NHL and B-CLL, there remain a substantial number of patients with B-cell malignancies who exhibit primary resistance to, or relapse after, optimal chemotherapy. A therapy that is effective over long periods of time in most or all patients with B-cell malignancies is desired.